The preparation of coatings by depositing film-forming materials on electrically conductive substrates under the influence of an applied voltage is known and is finding ever increasing acceptance in technology for reasons of quality and rationalization. The water-thinning lacquers used in that application are called electrophoresis lacquers or EC lacquers. They are characterized by a good grip and very good protection against corrosion in thin coats, and are prepared in practice from epoxy resins on a basis of bisphenol A and amines and/or amino alcohols. Water solubility is achieved at least in part by forming salts on tertiary amino groups. Thus a basic group is preserved in the baked-on film, which can lead to quality degradation on exposure to a sufficiently great humidity.
Thus, in DE-A-33 36 749=EP 137 459, an amino epoxide resin free of epoxide groups and containing urea groups is prepared by increasing the chain length of an amino epoxide resin containing epoxy groups with an amide or urea derivative containing two amino groups and combining it with crosslinking agents capable of transesterification. The tendencies toward improvement of second-coat adhesion and stone impact strength, which are then recognizable, are not, however, sufficient to satisfy today's more severe requirements. The tertiary amino groups contained in the resin system require a pH of 5 to 6 for solubility.
Synthetic resins having reactive amino groups can be prepared from resins bearing epoxide groups generally by two methods, namely by reaction with polyaminoketimines or with polyamines in excess.
In DE-A-22 52 536 and DE-A-22 65 195 amino epoxide resins are described which are formed by the reaction of epoxide resins with polyamides, which in addition to at least one secondary amino group-, contain primary amino groups latently protected by ketimine groups. For the thermal crosslinking of the films the resins are either mixed with blocked polyisocyanates or are reacted with partially blocked polyisocyanates to a self-crosslinking binding agent. On the basis of this method of preparation, at least one tertiary amino group per molecule is present after the lacquer is baked on, which can negatively affect the properties of the film. To prevent undesirable secondary reactions it is necessary to set out from freshly prepared ketimine, and it is essential to maintain anhydrous conditions of reaction.
In DE-A-31 09 282, isocyanate-reactive functional groups of an epoxide resin (component A) are linked through a diisocyanate (component C) with a mainly aliphatic polyamino compound (component B) whose primary amino groups have been converted to ketimino groups to form a resin of type A-C-B. At the same time numerous byproducts are formed, which have different properties and therefore interfere with uniform eledtrophoretic coating.
In DE 33 25 061 and DE 35 19 935, epoxide resins are reacted with primary diamines whose amino groups are protected by ketimine groups. Chain lengthening through the secondary amino groups is not described. The products that form are not uniform.
In EP-4090 resins containing amino groups are prepared which, at an amine number of 30-150, contain at least 1/3 primary and/or secondary amino groups and are reacted with crosslinking agents which contain reactive ester groups. The resins are prepared by the reaction of epoxide resins with an epoxide equivalent weight of 500 to 2,000 with excess polyamine, which is distilled out. On account of the high viscosity of the resin, severe frothing problems occur in the distillation, making impossible any complete removal of the unreacted polyamines.
In DE-A-27 37 375 is described the reaction of epoxide resins with an epoxide equivalent weight of 400 to 4,000 with excess polyamines which contain at least two amine nitrogen atoms and at least three amine hydrogen atoms per molecule, and the unreacted polyamine is distilled out. To control the amine number and elasticity the molecule is modified with monocarboxylic acid or monoepoxides. The crosslinking is performed with formaldehyde condensation resins.
In EP-189 727 and EP-189 729 the epoxy resin is additionally modified with polyalcohols and crosslinked with blocked polyisocyanates. By this method dark yellow to brown colored plastics are obtained, which still contain 3,000 to 20,000 ppm of unreacted polyamine. Films are obtained which provide a protection against corrosion which no longer satisfies the increasingly severe requirements.
In U.S. Pat. No. 3,963,663 an aminoepoxide resin containing urethane groups and prepared preferentially with primary diamines on a polypropylene glycol basis (=Jeffamin 400) is crosslinked with phenolic resins. The urethane groups are introduced by the reaction of diisocyanates with hydroxy groups of epoxide resins of higher molecular weight. According to the Example, a useful product is obtained only with diamines of high molecular weight containing ether groups, because any excess amine that might possibly have to be distilled out cannot be completely removed on account of the high resin viscosities, and the lacquer qualities are impaired.
The demands of the market are now calling for lower solvent content and lower bake-on temperatures without loss of the grip and corrosion protection properties. Special value is today laid on increased stone impact strength and better properties of adhesion between coats.